The concentration of membrane cholesterol in normal or leukemic lymphoid cells can be manipulated by subjecting the cells in vitro to oxygenated derivatives of cholesterol, which specifically depress the synthesis of sterols. Alternatively, whole animals are treated with such oxygenated sterols in the diet. The effect of such treatments upon specific immunological functions of lymphocytes, such as target-cell killing by T-cells and antibody production by B-cells will be investigated. In addition, the effects of dietary administration of oxygenated sterols, putative contaminants of the human diet, on cellular nutrition will be studied. Also, the morphological and physiological changes occurring in lymphoid tissues (lymph nodes, spleen, thymus) occurring upon dietary and parental administration of oxygenated sterols, will be characterized in young and old mice.